Gregory (D-Amityville) will formally make the announcement for the 2016 election in an email to supporters Wednesday. He is expected to be nominated Wednesday night by county Democrats for another two-year term as a county lawmaker, a post he would leave early if he wins the House seat.

"There are times in my life when I have felt a call to service," Gregory said, alluding to his enlistment in the U.S. Army, where he rose to the rank of lieutenant. "I feel that call to service now because Long Islanders are tired of the bickering, partisanship and gridlock in Washington. They want to send someone with a record of working across party lines to get the job done."

Gregory, 45, called the contest "a competitive race," but said King's "rhetoric does not match his record," in Congress.

"He has voted to privatize Medicare, cut Social Security and restrict a woman's right to to make personal medical decisions," Gregory said. "He's out of touch with voters in the district."

Asked about Gregory's comments, King, of Seaford, who chairs the House Subcommittee on Counterterrorism and Intelligence, said he's proud to make the race about his record.

"I'm a conservative Republican, but I've fought hard for Long Island and worked with Democrats like [Suffolk County Executive] Steve Bellone and [Babylon Supervisor] Rich Schaffer," on issues including superstorm Sandy aid and federal funding for local school districts with large numbers of immigrant students.

Experts in both parties say Gregory faces an uphill fight against King, 71, who in 2014 won re-election with 68.5 percent of the vote -- the largest margin of any Long Island member of Congress.

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But King has challenges. His district is now 70 percent in Suffolk and next year is a presidential election and a large turnout often helps local Democrats. Democrats also have a slight enrollment edge -- 157,194 to 155,387 registered Republicans in the 2nd Congressional District. The Conservative Party, which always has backed King, has 9,168 voters.

Michael Dawidziak, a political consultant who works mainly for Republicans, said King will be hard to unseat because is so well known and has a "very Reaganesque" appeal.

"He's very outspoken and even if you disagree with him on certain issues, people like that he's very direct," he said.

Some Democrats expressed concern that Gregory's announcement may intensify a GOP effort to find an opponent to challenge him for the legislature this fall. Republicans have not run anyone against Gregory in his past two races.

John Jay LaValle, Suffolk Republican chairman, said "it's always odd to stand before voters and say you want to serve another two years and at the same time say you want to leave."

Tony Pancella, Babylon GOP chairman, declined to say if the town party had a candidate to run against Gregory, but said his announcement makes a challenge more likely. "I feel very protective of Rep. King. He's a great congressman and a national figure," Pancella said. "Anyone who is looking to take on the congressman is looking to take us on."

Gregory, who first won in a 2008 special election, discounted potential backlash.

"I have a strong record representing the 15th district," he said. "They know my priority is serving them ... whether it's at the county or federal level."

Some Democrats see Gregory's early entry as a way to pre-empt other Democratic House challengers should King's long-shot presidential bid gain traction or if he becomes a contender for vice president. King, has said he is testing the waters in the presidential race to raise the national security issue, but intends to run for re-election next year.

Gregory declined to say how much he will need to raise to wage a competitive race against King, who has $2.9 million in his coffers. He said he is starting early because "it's a big district and it takes a lot of time to know everyone from Nassau County to Islip."